Slice a keyed list: fold and walk over a range of keys

A fold whose kind has PURE_FOLD_SLICED (128), and a walk whose argument
has PURE_EACH_SLICED (bit 0), first take two numbers, a low and a high
key, and run for the items of their list (keyed by a number) whose keys
are from low up to before high, found by key: the range is the program's
meaning, not an optimization, so their blocks may fault (items out of the
slice never run). A fold a leaf at a time now leaves out the lanes of a
leaf outside the items it runs for, and a leaf that would fault runs item
by item over those items alone. Slices are neither shaped nor given
automatic ranges.

ranges now checks a count over a slice whose test divides (a division by
zero failing it only when inside the slice) against a model, the same on
the item-by-item path, and a walk over a slice.

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